A new peptide containing immunoreactivity to bombesin was discovered in bovine milk, termed BMB. This peptide chemically differed from both the amphibian peptide and another mammalian bombesin-related peptide, GRP, isolated from porcine non-antral stomach tissue. Preliminary bioactivity studies further determined its differences from existing bombesin peptides, although it belongs to that family of peptides: BMB exhibited a bombesin-specific contraction of isolated rat uterus and guinea-pig large intestine and had a particularly evident effect on rhythmic movement. It was estimated to be 25-40% as active as bombesin and more potent than GRP.